Fix: KPI List of available metrics

Implementation of KPIServiceImpl#listExecutions returns Executions where user has direct READ permission assigned.

Change this to List<Execution> with @PostFilter("hasPermission(returnObject, ...

List of available metrics

GET /api/v1/kpi/observations returns the list of names of available metrics:

[ "accession.updates","accessions.count","accessions.doi","dataset.accessions","institute.pdci"... ]